QFN Rework: No-Clean or Water Soluble Flux?

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QFN Rework: No-Clean or Water Soluble Flux?

We are replacing 32 lead QFN and 14 lead DFN components. We can use either a no-clean or a water soluble process. Which do you suggest?
Board Talk

Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes

Phil Zarrow
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.

Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

Transcript

Phil
And welcome to Board Talk. This is Phil Zarrow and Jim Hall of ITM Consulting here to discuss with you SMT process, methodologies and materials. And I do believe we have exactly that kind of a question today.

Jim
This comes from M.D. No clean or water soluble for QFN components. We're replacing 32 lead QFN and 14 lead DFN components. We can use either a no clean or a water soluble process. Which do you suggest?

Is there a formula or a standard that explains when to use a no-clean or water soluble process with these component types? Well, I don't know of any formula or standard, it's certainly done both ways.

Ideally, it would be wonderful to just use the no clean and assume the residues are safe and not have to go to a cleaning process.

Phil
In many cases and in many applications that is the case. There are risks involved, particularly when we talk about QFNs.

Jim
A paper published by our favorite cleaning gurus, Terry Munson from Foresight showed that they removed QFNs that had been processed with no clean and found active residue under the paste.

Terry theorized in his paper that the component and all of the paste under it had in fact seen the proper time/temperature profile during the re-flow process should have deactivated the paste.

But, because of the configuration of the QFN, with a large amount of solder paste in the center for the thermal pad, what was actually happening was the flux residue around the perimeter of the package was hardening sooner than the center, preventing the evaporation or the escape of the evaporated materials from the no-clean paste.

Remember, no cleans are deactivated by three mechanisms: chemical reaction of the components, evaporation, and encapsulation. So even if we have the proper cycle, if you suppress that evaporation and trap those materials, you have the potential of having some corrosive materials. So my instinct is to use a water soluble process.

You need to make sure that you use a saponifier, because in most cases the high surface tension of pure water alone will not penetrate under the QFNs to clean out all of the residues. And with a water soluble you must remove the residue.

Phil
So probably the best of all courses for QFN component rework is to use no-clean flux, but clean the no-clean flux using an engineered aqueous saponifier. As our other favorite cleaning guru, mild mannered Mike Bixenman from Kyzen, will tell you, you must be certain that you match the solvent - in this case the engineered aqueous, to the soil, and the residue of the no clean.

Jim
I absolutely agree with that, but it's always been my feeling, if I know I'm going to clean, why not use the water soluble which is designed to be clean, and not a no clean which is designed not to be cleaned?

Now it's done all the time, but it seems to me if you've got this flexibility and you're going to end up with a cleaning process with an engineered chemistry and some sort of saponifier, that it always seemed more logical to me to go with a water soluble that's designed to be clean.

But check it out for yourself. Make sure you run the tests on your parts and your components with your solder paste.

Phil
I suspect we'll get a bunch of mail on this one, Jim.

Jim
But whatever you do, don't solder like my brother.

Phil
And don't solder like my brother.

Comments

One caution about water-soluble fluxes: They can leave salts behind, which can cause leakage problems in high-impedance circuits. We once had to scrap a whole run of boards that had been processed with water-soluble flux, because we simply could not get all the salts off. Sending them back for re-cleaning did not help. The bias voltages were all screwed up. Changed to no-clean, and the problems went away.

The circuit's impedances weren't even all that high. I believe the largest resistor was less than 10 Meg ohms. So it was far from a picoampere-level design.

The difference between solvent-cleaning after no-clean (or RMA) process, versus water-cleaning after water-soluble flux process is the difference between organic and inorganic compounds. The former do not form ionic salts, while the latter do. In my experience, water-soluble process is inappropriate for any high-impedance analog circuit.

Marc Stewart, JTS Consulting, USA

I am fairly new to the electronic world with only dealing with PCBA CMs and manufacturing for the last 5 years. I recently switch businesses inside GE from Appliance (non complex, single sided boards) to the Smart Meter (complex, double sided boards) and in Appliance we have been using no clean process at all our CMs and that is what I have my training and experience with on resolving Issues.

Since moving here to Smart Meters, I have been pushing to reduce cost by moving and testing no clean process at current CMs. I recently had issue as described in this article of residue left under the QFN package with a cleaning process. The resolution was a series of DOE at the CM to determine the best line speed, water temperature, and pressure to run the boards through the process and add saponifier.

My question is there a continuing trend to move to no clean, or is the industry in agreement that certain technology will have to remain in a cleaning process?

Charles Danner Senior Supplier Quality Engineer GE Energy Digital Energy